1. Field of the Invention
The present invention relates to a multi-port coupler for use in an optical fiber amplifier, a fiber laser, or the like for optical communications, particularly for short distance transmission systems, and an optical amplifier and a fiber laser using the same.
Priority is claimed on Japanese Patent Application No. 2006-149696 filed on May 30, 2006 and Japanese Patent Application No. 2006-260881 filed on Sep. 26, 2006, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In general, a cladding pump structure is adopted in a high power optical amplifier, a fiber laser, and the like. This cladding pump structure supplies pumping light propagating through a cladding in order to amplify light propagating through a core of an optical fiber. The cladding pump structure is widely adopted and simultaneously a high power laser diode (hereinafter, referred to as LD) for multi-mode fiber output is used. A cladding pump fiber uses a double cladding structure having multiple cladding layers arranged around a rare earth element doped core.
To introduce light of a high power multi-mode LD into the cladding pump fiber, a multi-port coupler is used. The multi-port coupler has a function for coupling light of at least two multi-mode fibers to the cladding pump fiber and simultaneously connecting respective cores of the cladding pump fiber and a single-mode optical fiber (hereinafter, referred to as SM fiber) through which a conventional optical signal passes. The function of the multi-port coupler can obtain high power by connecting a number of LDs respectively having the power of only several watts (W) to the cladding pump fiber. A structure of an optical amplifier using the multi-port coupler is illustrated in FIG. 1.
An optical amplifier 21 of FIG. 1 is constructed to include a multi-port coupler 22 into which signal light and pumping light are incident and a cladding pump fiber 23 whose end is connected to an output terminal of the multi-port coupler 22. In the multi-port coupler 22, a signal fiber 25 constructed with a central SM fiber and pumping fibers 24 constructed with multi-mode fibers arranged around the signal fiber 25 are unified. A front side of the multi-port coupler 22 is reduced in diameter. The signal light and the pumping light can be incident into the cladding pump fiber 23 for optical amplification connected to a reduced-diameter output terminal of the multi-port coupler 22. Although not illustrated in FIG. 1, a signal light source is connected to the signal fiber 25 coupled to the multi-port coupler 22. LDs 10 are connected to the plurality of pumping fibers 24, respectively. The optical amplifier 21 excites rare earth ions doped into the core of the multi-port coupler 22 by propagating the pumping light (for example, a wavelength of 910-980 nm) to the cladding of the cladding pump fiber 23 through the multi-port coupler 22. The optical amplifier 21 amplifies incident signal light by propagating the signal light to the core of the cladding pump fiber 23 through the multi-port coupler 22, such that the amplified signal light (or high power signal) is output from the cladding pump fiber 23. In this type of optical amplifier, the gain level is 20 dB or more and the maximum power is 1 W-1 kW.
However, the lifetime of LDs 10 should be prolonged to improve reliability in a conventionally used optical amplifier in which a multi-port coupler and a cladding pump fiber are combined.
To prolong the lifetime of LDs, not only a high reliability design of the LD itself but also temperature management as a use condition is important. However, in the conventional optical amplifier, the LDs may suddenly fail according to use condition, thereby adversely affecting high reliability of the overall optical amplifier.
From the keen examination by the applicants, it has been found that an pump LD is suddenly damaged since high power signal light output from the optical amplifier returns from an external reflection point to the optical amplifier. Since the reflected light is inversely amplified while passing through the core of the cladding pump fiber and also light leaked into the cladding reaches the pump LD due to splice loss occurring in a splicing point between the cladding pump fiber and the multi-port coupler, the pump LD is damaged, thereby causing the failure. In particular, when the gain of the optical amplifier is 20 dB or more, the power of the light which is passed through the optical amplifier reaches more than 100% of it's own output power. When this strong reflected light, whose power can be ten times higher than that of the pump LD itself, goes backward into the LD, the LD is damaged.
To address this problem, the applicants have conducted research on a structure for preventing the pump LD from being damaged even when the reflected light returns to the optical amplifier.
On the other hand, an optical isolator is conventionally used in conventional technologies for suppressing the reflected light itself. However, there is a problem in that the optical isolator may attenuate the reflectance to only about −20 dB and also the optical isolator for light of several watts (W) or more is too expensive.
Since the reflected light propagates inside the core of the amplification fiber, the reflected light is not emitted to the pump LD as long as the light is not leaked from the core. According to the examination on how the reflected light is incident into the LD, it has been found that the reflected light is emitted to the pump LD due to the main cause of splice loss of the core caused at the splicing point of the multi-port coupler and the cladding pump fiber.
The loss in the splicing point occurs since core diameters (or intensity distribution sizes) of two fibers are very different from each other. In general, the high power cladding pump fiber has the core diameter of 20 μm or more. On the other hand, the core diameter of an SM fiber used for signal propagation is about 5 μm. For this reason, the magnitude of the loss in the splicing point becomes 5 dB or more, particularly in a propagation direction of the reflected light.
The reason why the core diameter of the cladding pump fiber is large is that the optical power density is very large in a high power amplifier and the optical fiber is affected by the non-linear optical effect. To avoid this influence, the core diameter of the cladding pump fiber is increased and consequently the low optical energy density in the fiber is designed. However, since multi-mode transmission is difficult and also bend loss is large in the core whose sectional area is large, the core is not proper for the signal fiber. Accordingly, it is desirable that the transmission fiber has a core whose sectional area is small and the amplification fiber has a core whose sectional area is large. Since the multi-port coupler is arranged between these two fibers, splice loss may be reduced to a certain degree if the sectional area of the core of the multi-port coupler has a middle size between the sizes of the sectional areas of the two fibers. It is obvious that the structure of the multi-port coupler increases the splice loss without decreasing the splice loss, since an outer diameter of a fiber in the multi-pump coupler is reduced to conventionally couple multiple fibers to one fiber.
FIGS. 2A and 2B illustrate a splicing point of the conventional multi-port coupler 22 and the cladding pump fiber 23 shown in FIG. 1. A front end of the multi-port coupler 22 connected to the cladding pump fiber 23 unifies a central signal fiber 25 having a core 26 and a plurality of pumping fibers 24 around the signal fiber 25, and has a structure in which the multi-port coupler 22 is reduced in diameter in a tapered shape toward an end so that an outer diameter of the multi-port coupler 22 can be the same as that of the cladding pump fiber 23. In the front end, the diameter of the core of the central signal fiber 25 is further reduced. When the cladding pump coupler 23 whose core has a large diameter is connected, a difference between the core diameters of fibers at both sides is further increased.
When the above situations are considered, it is actually difficult to reduce splice loss occurring in the splicing point of the multi-port coupler.